Apparatus for producing low-density pellets from slurries containing film-forming materials



1970 F. J. ZAVASNIK 3534,43

APPARATUS FOR PRODUCING LOW-DENSITY PELLETS FROM SLURRIES CONTAININGFILM-FORMING MATERIALS Original Filed July IL 1966 REACTOR INVEN TOR F.J. ZAVASNIK A T TORNE YS United States Patent APPARATUS FOR PnoiiUcrNoLOW-DENSITY PELLETS FROM SLURRIES CUNTAINING FILlVI-FORMING MATERIALSFred J. Zavasnik, Bartlesville, 0kla., assignor to Phillips PetroleumCompany, a corporation of Delaware Original application July 11, 1966,Ser. No. 564,406, now Patent No. 3,432,579, filed Mar. 11, 1969. Dividedand this application Aug. 1, 1968, Ser. No. 749,368

Int. Cl. B29f 3/12 US. Cl. 18-1 3 Claims ABSTRACT OF THE DISCLOSURE Theapplication is a division of application Ser. No. 564,406, filed July11, 1966 and now Pat. No. 3,432,579. This invention relates to anapparatus for producing low-density pellets from a slurry offilm-forming ma- 7 terial. In one aspect this invention relates toproducing hollow pellets from a solution of a film-forming materialdissolved in a solvent compatible therewith and precipitated by coolingto form a precipitated polymer-solvent slurry. In another aspect thisinvention relates to a fluidized drying system for producing low-densitypolyethlene pellets. In yet another aspect the invention relates toproducing a resinous material having a low bulk density. In yet anotheraspect this invention relates to novel low-density pellets composed offilm-forming material.

The formation of particles by the solution polymerization offilm-forming materials is well known under the prior art. A solutionpolymerization is defined as the polymerization of a momomer, whereinthe monomer is dispersed in a solvent different from the monomeremployed but neverthless compatible therewith for dissolving the formedpolymer. In such systems, the polymer is recovered from the solutionthrough cooling and depressurization whereby the solvent is flashedtherefrom. If the depressurization is sufficiently low and noatomization occurs, the polymer product foams and cools into a mass ofhoneycomb or cellular type material which is difficult to process. Insuch a system it has heretofore been quite diflicult to prepare polymerparticles having a low bulk density. Such a low bulk density materialwould be useful as in a pourable insulation, as additives to paperslurry in the paper-making industry and'as additives in foamed sheetsand in insulated products.

It, therefore, is an object of the present invention to provide anappartus and method for producing separate and discrete low-densitypellets which have a thin strong skin and which are formed from afilm-forming material.

It is a further object of this invention to provide a novel fluidizeddrying system for producing low-density pellets of polyethylene.

It is a still further object of this invention to provide for forminglow bulk density resinous material.

Yet another object of this invention is to provide a low bulk densityresinous material.

Other objects and many of the attendant advantages of this inventionwill be readily appreciated as the same becomes better understood byreference to the following ice detailed description which is to beconsidered in connection with the accompanying drawings wherein:

FIG. 1 is a schematic description fo the various steps of my process; 1

FIG. 2 is a perspective view of the novel fluidized drying apparatus;and

FIG. 3 is a section taken along the lines 3-3 of FIG. 2.

Briefly, this invention comprises the steps of forming a pressurizedsolution comprising a volatile solvent having dissolved thereinfilm-forming polymeric material; cooling the polymer solution to causeprecipitation of polymeric film-forming material from the solutionthereby forming a slurry; subdividing the said slurry of film intopellets; subjecting the pellets to a drying temperature for a timesuflicient to cause partial volatilization of the said solvent from thepellets at their surface to produce a self-sustaining structure; anddrying the pellets in a fluidized medium at a temperature sufiicient tocause shrinkage along their outer surfaces, thereby causing the pelletsto burst because of the vaporization of the solvent entrapped therein,and recovering the dried pellets of polymeric materials.

The invention is applicable to the formation of spheroidal-shapedparticles which can be obtained from any natural or syntheticfilm-forming material that is soluble in either an acidic, alkaline orneutral aqueous solution or in an organic solvent; that can form asolution whose viscosity is sufliciently low to permit subdivision intosmall pellets; that can be capable of gelling on evaporation of solventtherefrom to form a relatively tough gas-impermeable skin or film.

The term film-forming material is used herein to refer to film-formingmaterials as a class. The organic materials include cellulousederivatives, such as cellulose acetate, cellulose acetate-butyrate, andcellulose acetatepropionate, thermoplastic synthetic resins, such aspolyvinyl resins, i.e., polyvinyl alcohol (wateror organicsolvent-soluble), polyvinyl chloride, copolymers of vinyl chloride andvinyl acetate, polyvinyl butyral, polystyrene, polyvinylidene chloride,acrylic resins such as polymethyl methacrylate, polyallyl, polyethylene,and polyamide (nylon) resins, and thermosetting resins in thethermoplastic wateror organic solvent-soluble stage of partialpolymerization, the resins being converted after or during formation ofthe particles into a more or less fully polymerized solvent-insolublestages, such as alkyl, alkyd, polysiloxane, phenyl-formaldehyde,urea-formaldehyde and melamine-formaldehdye resins. All these resins arefilm-forming and, therefore, capable of forming tough-skinned particles.Natural film-forming materials are also included in the scope of thegeneral term including soybean protein, zein protein, alginates, andcellulose in solution as cellulose Xanthate or cuprammonium cellulos.Inordanic film-forming substances such as the sodium silicates,olyborates and polyphosphates are also contemplated as within the scopeof the above term.

The solvent employed will, of course, be dictated by the solubility ofthe film-forming material used. The solvent should, upon evaporation, beconducive to gelation of the material, so that a tough skin is quicklyformed over the surface of the pellet. Water, alcohols, ethers, esters,esters, organic acids, hydrocarbons and chlorinated hydrocarbons are themost noteworthy satisfactory solvents.

More particularly, the polymers applicable to this invention arehomopolymers and copolymers of two or more l-olefins having no branchingnearer the double bond than the 4-position, preferably having from 2 to8 carbon atoms per molecule, which a specific gravity of at least 0.94at 20 C.

Examples of such l-olefins include ethylene, propylene, l-butene,l-pentene, l-hexene, l-octene, 4-methyl-1-pen- 3 tene,4-methyl-l-hexene, 4-ethyl-1-hexene, S-methyl-lheptene,S-methyl-l-heptene.

The preferred solvents employed in forming the solutions of thisinvention are generally hydrocarbons, preferably paraflins and/ orcycloparaflins, having from 3 to 12 carbon atoms per molecule.Generally, any hydrocarbon which is relatively inert, nondeleterious,liquid under the conditions of operation of the polymerization process,and more importantly, a solvent for the polymer formed, can be utilized.

Examples of such solvents include propane, isobutane, normal pentane,iso-pentane, isooctane (2,2,4-trimethylpentane), cyclohexane,methylcyclohexan The solutions of this invention will generally containfrom about 1 to 20, preferably from about 5 to about 12, weight percentpolymeric film-forming material based upon the total weight of thesolution.

As stated hereinabove, any polymerization process which forms a solutionwhich is capable of gelling on evaporation to form a relatively tough,gas-impermeable skin or film is suitable for this process. A preferredmode of carrying out the process of this invention is illustrated inFIG. 1. A solution of polyethylene dissolved in cyclo hexane is preparedin reactor 1 according to the process described in U.S. Pat. 2,825,721.This reaction solution is transferred via conduit 3 to a flashing zone 5wherein any unreacted ethylene is flashed from the reaction solutionthrough conduit 7 and recycled back to the reactor feed (not shown).Generally, the temperature of the polymer solution in reactor 1 ismaintained at from about 200 to about 400 F. and a pressure of fromabout 350 to about 550 p.s.i.g. The polyethylene solution passes fromthe flash zone 5 via conduit 9 through a pressurized cooling means 11into an intermittent pressure letdown valve means 13.

Cooling means 11 can be a jacketed A2- to 2-inch diameter pipe from 10to 60 or more feet long which has cooling liquid such as water passingthrough its jacket. The temperature in cooling means 11 is maintainedsuch that the polymer solution passing through is cooled at least to thetemperature at which precipitation of polymer from the solution starts,i.e., a temperature at which polymer is no longer readily soluble in thesolvent. Although this temperature can vary depending upon the polymer,the solution, or the concentration of polymer in the solvent, it willgenerally be in the range of from about 190 to about 230 F. The pressurein cooling means 11 will be maintained sufliciently elevated so thatsubstantial vaporization of the solvent is prevented but lower than thepressure in the reactor 1 so that the pressure in that container is themotivating force for movement of the material through the cooling means11. This pressure can vary widely but generally will be from about 200to about 300 p.s.i.g.

The pressure letdown means 13 can be a conventional metering device inwhich a fixed amount of material is allowed periodically to passtherethrough. Such a metering device is fully and completely disclosedin US. Pat. 3,167,398, which employs the captive ball concept wherein achamber is provided having inlet and outlet orifices at opposite endsthereof and through which the polymer slurry from reactor 1 must pass inorder to reach the generally cylindrical and upright initial drying zone15. This zone 15 is maintained at a temperature and pressuresufiiciently lower than cooling means 11 in order to cause vaporizationof the solvent. Generally, the pressure in zone 15 is substantially orslightly less than atmospheric.

With reference to FIGS. 2 and 3, wherein the fluidized drying zone isshown in greater detail, the precipitated slurry is passed from theletdown valve 13 via conduit 9 into a hopper 17 which is positioned inthe top of zone 15. The hopper contains a spreading means 18 forspreading the precipitated slurry into a plurality of mold cavities 19which extend through a drying plate 21 positioned underneath the hopper.This spreading means can be any conventional device such as a roll r, ascraper, etc.

A drying fluid is passed via conduit 23 up through the initial dryingzone 15 and passes through the material in the mold cavities 19 in thetop of drying plate 21 and out through the exhaust conduit 25. Anydrying fluid which is inert towards the precipitated polymer, 'i.e.,air, nitrogen, ethylene, or the like, can be employed for this dryingoperation. The drying fluid is maintained at a temperature substantiallyaround 210 F. A baflle means 20 is positioned underneath the hopper 17to prevent drying fluid from passing up into the hopper and to preventthe polymer slurry from falling out of the mold cavities 19 during theloading thereof.

The drying plate 21 is rotated in incremental steps from the fillerposition underneath the hopper to an ejection position underneath apellet ejector mechanism 29 in a time period sufficient to evaporate thesolvent from the exposed surface of the pellets. As the pellets aredried, they tend to shrink thereby permitting circulation of the dryingmedium through the mold cavities 19. This period will depend on theparticular solvents and film forming materials employed but will usuallyrange from 210 minutes. This time rotation of the drying plate iscontrolled by a timing and indexing means 27 which is also operativelyconnected to the pellet ejector 29 whereby the partially dried pelletsare pushed into a chamber 31, which forms a closed section ofcylindrical drying zone 15. Chamber 31 is provided with heating means(not shown) which may be of any suitable nature for pulsing a fluidmedium, which can be the same as described hereinabove, through conduit33 and upwardly through closed chamber 31. The temperature of thisfluidized medium is adjusted according to the stability and softeningpoint of the particular film-forming material, the size of the pelletsproduced, and the stability of the solvent employed. However, as thoseskilled in the art appreciate, because the cooling effect ofevaporation, a very high temperature may be used without injury tolow-melting or easily decomposable materials. As the pellets areintroduced into the chamber 31 from the conduits in plate 21, they beginto swell and become buoyant. As they become more bouyant, they arecarried gradually upwardly in the turbulence produced by pulsing theflow of drying medium in chamber 31. The pellets are thus caused to riseprogressively in the chamber 31 until they reach the level of thechamber at which they find the outlet 37. The pellets then exit throughthe outlet and are recovered by suitable recovery means not shown.

In order to keep the pellets from falling to the bottom of chamber 34,the diameter of the chamber is smaller at the bottom than at the top andglass beads 36, which prevent the pellets from adhering together duringthe initial stages of fluidized drying, are positioned on a 40-mesh 0.1standard screen 35 at the point where chamber 31 is joined with conduit33.

The following example is oifered to further illustrate the invention.

EXAMPLE A solution comprising cyclohexane containing 8 weight percentpolyethylene, based upon the total weight of the solution, and having adensity of 0.96 and a melt index of 0.2 was passed from reactor 1 viaconduit 3 into a flash zone 5 where unreacted ethylene was flashedthrough conduit 7. The polyethylene-cyclohexane solution was then passedvia conduit 9 through cooling means 11 which was composed of a /2-inchdiameter copper tubing about 20 feet long. This tubing had a waterjacket thereabout through which was circulated cooling water at atemperature of about F. The polyethylene polymer precipitated from thesolution and formed a slurry. The polyethylene slurry was passed fromcooling means 11 via conduit 9 into a pressure letdown valve 13 whichpermitted the passage of about 9 pounds per hour of slurry throughconduit 9 into the hopper 17 of the top zone 15. The slurry was spreadinto the mold cavities 19 in drying plate 21. The drying plate was timedto rotate from the filling position under hopper 1'7 over to the ejectorplate 29 in twominute cycles. Drying air at a temperature of 210 F waspassed up through drying zone 15 and through the mold cavities 19 inplate 21. The partially dried pellets were ejected from the conduits 19by ejector 29 into a chamber 31. A drying air at a temperature of 210 F.was pulsed upwardly through chamber 31 at a pulsation frequency of 60cycles a second which was sufficient to agitate the pellets falling intochamber 31. This procedure was maintained at ten-minute intervals atwhich time the pellets had 0.3 percent solvent and a minimum bulkdensity of 6 pounds per cubic foot. The polyethylene pellets were blowninto conduit 37 where they were collected.

1 claim:

1. An apparatus for producing low bulk density resinous pelletscomprising a generally cylindrical and upright heating chamber; meanslocated in the top of said chambet for subdividing a polymer slurry intoindividual pellets; means operatively connected with said subdividingmeans for drying the surface of said pellets; means for ejecting saidpellets from said surface drying means into said heat chamber; means forpulsing a fluid medium into said chamber in order to agitate the pelletsand cause vaporization of the solvent entrapped inside of the pellets;and recovery means located in the wall of said chamber for recoveringthe low bulk density dried pellets from said heating chamber.

2. Apparatus according to claim 1 characterized in that the subdividingmeans consists of a plate having a plurality of mold cavities in the topthereof, in combination with a roller for pushing the polymer slurryinto the mold cavities.

3. A drying apparatus according to claim 1 in combination with a meansfor maintaining a source of heated polymer solution under pressure, acooling means for precipitating the polymer from solution to form aslurry and delivery means for delivering said slurry to said dryingapparatus.

References Cited WILBUR L. MCBAY,

U.S. C1. X.R.

Primary Examiner

